This invention relates to underground gasoline storage tanks, and more particularly to systems for controlling escape of gasoline vapor from such tanks.
During refueling of automobiles and other vehicles, liquid gasoline is delivered into the vehicle fuel tank, and a mixture of gasoline (or other fuel) vapor and air is displaced from the tank. To minimize escape of gasoline vapor into the atmosphere, gasoline dispenser nozzles are typically equipped (as often mandated by local environmental protection regulations) with vapor recovery vacuum systems to collect the displaced gasoline vapor, and air, and deliver it back into the ullage (i.e., vapor) space of the underground storage tank (xe2x80x9cUSTxe2x80x9d). Preferably, a 1-to-1 ratio balance is sought between volume of liquid gasoline drawn from the underground storage tank, e.g. during vehicle refueling, to volume of gasoline vapor and air returned into the ullage space by the vapor recovery system. However, due to a variety of factors, including, e.g., differences in temperature, inefficiencies in the vapor recovery system, ingestion of excessive external air, etc., such a balance is difficult to achieve. As a result, some amount of gasoline vapor may be discharged, or air ingested, through the UST pressure/vacuum relief vent valve during any 24-hour period of operation.
This problem has been addressed, in part, by design of ORVR (xe2x80x9conboard refueling vapor recoveryxe2x80x9d) equipped vehicles, in which gasoline vapor collecting in the ullage space of the vehicle tank is recovered onboard the vehicle, making it necessary for the fuel dispensing system to recover only a relatively smaller volume of gasoline vapor and air during refueling, e.g. as compared to non-ORVR vehicles. As a result of the differences between ORVR-equipped and non-ORVR-equipped vehicles, and the fact that both types of vehicles are in regular use, fuel dispensing systems must be designed to detect and accommodate different vapor recovery requirements.
One such fuel dispensing system employs the Healy 800 Nozzle, from Healy Systems, Inc., of Hudson, N.H., assignee of the present application, which is embodied in my earlier U.S. Pat. No. 6,095,204, issued Aug. 1, 2000, the complete disclosure of which is incorporated herein by reference. However, during ongoing field-testing of the Healy 800 Nozzle for purposes of addressing a need to prevent return of too much air when refueling ORVR-equipped vehicles, a troubling phenomenon has been uncovered. A feature of the Healy 800 Nozzle is that it reduces the volume of air returned to the underground storage tank to approximately 25% of the liquid volume dispensed to an ORVR-equipped vehicle. It has been discovered that this can create a problem in a busy service station because ORVR refueling can cause the vapor space pressure to fall to xe2x88x928.0 inches W.C. (xe2x80x9cwater columnxe2x80x9d), at which point the UST pressure/vacuum relief vent valve will open, thus introducing air into the UST. For example, calculations show that less than 600 gallons of gasoline dispensed to ORVR-equipped vehicles can reduce the UST pressure by +8.0 inches W.C. when the ullage space is 20,000 gallons. Additional fueling of ORVR-equipped vehicles beyond that point will then result in a one-to-one relationship of air returned to the UST versus liquid gasoline dispensed, as the Healy 800 Nozzle will continue to return air at a 25% rate while the pressure/vacuum relief vent valve will continue to reopen to allow inward air flow equal to 75% of the liquid gasoline dispensed. Later, when sales activity slows down in the evening and refueling of ORVR-equipped vehicles drops off, the large quantity of air previously ingested will promote evaporation of liquid gasoline into the air in the ullage space, as the enclosed system of gas and liquid moves toward an equilibrium of hydrocarbon concentration in the ullage space with the volume of liquid gasoline. The increasing concentration of gasoline vapor will cause the pressure in the UST to rise, potentially to a positive pressure of +3.0 inches W.C., which will cause the pressure/vacuum relief vent valve to reopen, releasing gasoline vapor into the environment. The problem is not apparent for service stations pumping an average of less than about 150,000 gallons per month; however, it can be very pronounced for larger sites, e.g. those that pump an average over about 500,000 gallons per month.
According to one aspect of the invention, a passive pressure control method for controlling pressure in ullage vapor space of a volatile liquid fuel underground storage tank (xe2x80x9cUSTxe2x80x9d) comprises the steps of: removing liquid fuel from the UST, including for delivery into a vehicle fuel tank, delivering into the ullage vapor space of the UST, to replace the volume of liquid fuel removed, a gaseous flow comprising at least one of: (a) fuel vapor and air, e.g. displaced from the fuel tank by delivery of the liquid fuel; and (b) air; and, during periods of increasing ullage vapor space pressure, allowing vapor to flow into an auxiliary vapor space of variable volume defined at least in part by a resilient wall member, the flow of vapor into the auxiliary vapor space causing deflection of the resilient wall member, thereby increasing the combined vapor storage volume of the ullage vapor space and the auxiliary vapor space.
Preferred embodiments of this aspect of the invention may include one or more of the following additional features. The passive pressure control method comprises the further step of, during periods of decreasing ullage vapor space pressure, causing vapor to flow from the auxiliary vapor space under pressure of deflection of the resilient wall member. The passive pressure control method comprises the further step of treating the gaseous flow into the ullage vapor space to increase the concentration of fuel vapor in the gaseous flow, including toward saturation.
According to another aspect of the invention, a passive pressure control system for controlling pressure in the ullage vapor space of a volatile liquid fuel underground storage tank (xe2x80x9cUSTxe2x80x9d) comprises means for temporarily, during periods of increasing ullage vapor space pressure, allowing vapor to flow into an auxiliary vapor space of variable volume, defined at least in part by a resilient wall member.
Preferred embodiments of this aspect of the invention may include one or more of the following additional features. The passive pressure control system further comprises means for temporarily, during periods of decreasing UST vapor space pressure, causing flow of vapor from the auxiliary vapor space into the ullage vapor space. The passive pressure control system further comprises means for treating a gaseous flow into the ullage vapor space in a manner to increase the fuel vapor concentration of the gaseous flow, including toward saturation.
According to still another aspect of the invention, a passive pressure control system for controlling pressure in the ullage vapor space of a volatile liquid fuel underground storage tank (xe2x80x9cUSTxe2x80x9d) comprises an underground storage tank defining a storage volume for storage of volatile liquid fuel with an ullage vapor space, an auxiliary tank defining an auxiliary vapor space in communication with the ullage vapor space, the auxiliary vapor space defined at least in part by a resilient wall member, the resilient wall member being adapted to deflect from an at-rest position in response to increasing vapor pressure in the auxiliary vapor space, thereby to increase the contained effective vapor storage volume of the auxiliary vapor space, and the resilient wall being adapted to return toward the at-rest position in response to decreasing vapor pressure in the auxiliary vapor space, thereby to decrease the contained effective vapor storage volume of the auxiliary vapor space.
Preferred embodiments of this aspect of the invention may include one or more of the following additional features. Deflection of the resilient wall member from the at-rest position in response to increasing pressure in the auxiliary vapor space increases the combined contained effective vapor storage volume of the ullage vapor space and the auxiliary vapor space, allowing vapor to flow from the ullage vapor space into the auxiliary vapor space. Return of the resilient wall member toward the at-rest position in response to decreasing pressure in the auxiliary vapor space decreases the combined contained effective vapor storage volume of the ullage vapor space and the auxiliary vapor space, causing vapor to flow from the auxiliary vapor space toward the ullage vapor space. The passive pressure control system further comprises a pressure relief vent valve in communication with the ullage vapor space and configured to open while pressure of vapor within the ullage vapor space exceeds a predetermined maximum pressure, thereby to permit release of vapor into the environment, wherein deflection of the resilient wall member of the auxiliary vapor space from the at-rest position in response to increasing pressure within the auxiliary vapor space serves to reduce the volume of vapor released to the atmosphere during normal operation. The passive pressure control system further comprises a vacuum pressure relief vent valve in communication with the ullage vapor space and configured to open while pressure of vapor within the ullage vapor space is below a predetermined minimum pressure, thereby to permit ingestion of air into the ullage vapor space, wherein return of the resilient wall member of the auxiliary vapor space toward the at-rest position in response to decreasing pressure within the auxiliary vapor space serves to reduce the volume of air ingested into the vapor space during normal operation. The auxiliary tank comprises a flexible bladder, e.g. a thin wall flexible urethane bladder, defining the resilient wall member. The bladder is disposed within a storage tank, preferably mounted about an inlet defined at an upper end of the storage tank. The auxiliary tank pressure/vacuum relief valve defines an orifice sized to limit flow of air out of the air space external to the bladder (and thus limit flow into the auxiliary vapor space) to a predetermined rate, thereby to restrict the rate of change of the air volume external to the bladder due to pressurization of the ullage vapor space. The relief valve defines an orifice sized to indirectly limit flow of air or vapor into and out of the auxiliary vapor space to a rate of about 2.5 gallons per minute. For use with a balance-type vapor recovery system, the vacuum relief valve is set to near atmospheric pressure and the pressure relief valve is set to near atmospheric pressure.
In preferred embodiments of each of the aspects of the invention described above, the gaseous flow may be treated or conditioned, e.g. by passing it through a liquid fuel mist chamber or through a fuel-wetted mesh, or by causing the gaseous flow to maintain extended flowing contact with liquid-gasoline-wetted surfaces, or by placing the gaseous flow in extended, close proximity or contact with liquid gasoline, e.g. by entraining the gaseous flow with a flow of liquid gasoline and/or by bubbling the gaseous flow through a body of liquid gasoline, e.g. in a liquid reservoir or in the UST itself. Controls may be provided, e.g., to actuate delivery of liquid fuel to the conditioning apparatus when gaseous flow is detected, and/or to ensure that the vacuum/pressure relief valve is not opened for flow of air until a flow of liquid fuel to the conditioning apparatus is confirmed. Effective vapor storage volume may also be increased by removal of vapor from the vapor space, e.g. for treatment.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.